This specification is common to related copending applications having Ser. Nos. 637,286; 637,841; 638,291 and which have the same assignee as this application.
The inventions disclosed herein pertain to a method and apparatus for storing digitized images on magnetic disk in real-time, with no delay between images in excess of the vertical blanking time.
The inventions will be illustrated herein in a system for recording and displaying a continuous screen of digitized X-ray images, but those skilled in the art will appreciate that the inventions are applicable to various systems for recording on a magnetic medium, such as a magnetic disk, digitized images that are generated in rapid order and images that are generated with a substantial time interval between them as well.
In digital fluorography, X-ray images emerging from the body are received by an image intensifier which converts them to optical images. The optical images are viewed by a video camera which converts them to analog video signals. The analog video signals for each horizontal scan line composing an image frame are digitized and the resulting digital data are usually stored, at least temporarily in a frame memory. Typically, images are processed in a digital video processor containing more than one full frame digital memory and an arithmetic logic unit so that one or more images in a sequence could be weighted and subtracted from another, for example, and the resulting image data stored. Before the inventions described herein were made, it had been necessary to reconvert the digital image data back to analog video signals for storage on analog magnetic disk. Analog video signal recorders had to be used because available digital disk recorders were not fast enough to record X-ray images produced at rates of thirty frames per second. One disadvantage of being constrained to use an analog video disk recorder is that it is necessary to reconvert the images back to digitized format before the images can be processed or treated again in the digital video processor circuitry. Re-digitization of the analog video signals introduced errors such as quantitization errors, noise errors and time base errors. The latter errors are a consequence of the images being produced and reproduced for display where the vertical sync rate of the video system is the time base while the images are stored on analog disk and retrieved from disk in reference to the time base of the disk drive. The errors mentioned, among other things, reduced the accuracy of registration between corresponding picture elements (pixels) in a sequence of images so, for example, sharpness of any subtracted images or otherwise reprocessed images was not optimal. Another disadvantage of the state of the art analog disk recorders is that they have limited storage capacity at tolerable signal-to-noise ratios and bandwidths. Still another disadvantage is that they allow a maximum pixel intensity range equivalent to about eight digital bits, that is, to about 256 intensity gradations, whereas certain processing techniques may generate data equivalent to up to twelve significant bits per pixel and some image acquisition modes generate 10-bit data per pixel. For analog storage of any of these images the data had to be compressed to eight bits.
Prior digital X-ray image processing systems using analog image storage techniques are described in U.S. Pat. No. 4,393,402 issued to Keyes, et al. and U.S. Pat. No. 4,449,195 issued to Andrews, et al. These patents are owned by the assignee of this application and their entire disclosures are incorporated herein by reference.
Digital disk recorders using Winchester drives have been available but were not fast enough for real-time recording of digitized images. The recording and retrieval rates of prior disk recorders have been limited by the fact that the pixel data had to be input to the write head in serial form so it could be written on one disk track after another on the same disk. Pixel data comes in too fast for being written without overlap.
A parallel input Winchester disk drive has recently been developed. It employs one drive and several disks on which parallel input data can be written simultaneously and read out simultaneously. The new drive is readily adaptable for increasing data storage rates in computer systems where data is generated and latched in storage until the disks are able to receive the data. However, in digital fluorographic applications, for example, a long sequence of image frames are frequently generated at the rate of thirty frames per second continuously with only a video blanking interval between them. When image data is being generated the disk drive must be available for recording on a continuous basis or parts of the data will be lost forever. The problems resulting from the time base of the video image acquisition system differing from the time base of the digital disk drive in the prior serial data input disk drive are still encountered in the recently available parallel data input disk drive.